Prior to 1969, the aircraft industry paid little concern to commercial jet aircraft engine noise. In Dec. 1969, the U.S. Federal Aviation Administration ("FAA") promulgated specific noise level regulations for aircraft under authority of public Law 90-411. Existing airplanes were required to be certificated for compliance with Federal Air Regulation No. 36 ("FAR 36"). Similar noise standards were prescribed by international civil aviation organizations (for example, "ICAO Annex 16"). Several states and municipalities also established maximum airport noise levels. Thus, it became imperative for airframe and engine manufactures and owners to take noise considerations into account in designing, building and using jet aircraft.
In the 1970's, The Boeing Co. conducted research to determine how noise levels could be reduced upon the take-off and landing of its various models of commercial airplanes. Boeing, of course, was concerned with design of future airplanes to meet more stringent noise standards, although it was also interested in modifications to currently operated aircraft to meet current noise standards.
With respect to its Model 727 aircraft with pratt & Whitney JT8D engines, Boeing proposed to treat the nacelles for the jet engines with sound-attenuating material. Boeing suggested adding the material to the inlet cowl, on some of the engine ducts and to the tailpipe. It was suggested that the treatment would reduce high-frequency fan noise, but would have virtually no effect on low-frequency jet rumble during take-off. Other modifications attempted by Boeing were reported to severely degrade airplane performance.
With respect to its Model 737 aircraft with Pratt & Whitney JT8D engines, Boeing proposed similar sound-attenuating treatment. Boeing further suggested using a mixer device to mix the air flow from the fan with the air flow through the engine to reduce the peak jet velocity and thus the jet noise from the engine. Boeing reported its test results in its "Summary 707-727-737-747 Noise and Emission Reduction Activities", April 1980, Document D6-40613-K.
Thereafter, the major airplane manufacturers and others continued substantial research effort toward developing retrofit kits for existing airplanes to attempt to meet FAR 36 noise level requirements. However, it was difficult to develop a kit which would meet FAR 36, Stage 2, requirements without, at the same time, degrading aircraft performance, increasing fuel consumption and unreasonably increasing costs. Although the effective date of the FAR 36 noise requirement was extended on several occasions, and the requirement was modified, FAR 36, Stage 2, went into effect on JUN. 1 , 1985. Implementation of that regulation affected the usefulness and value of many existing aircraft, including particularly Douglas DC-8 airplanes with Pratt & Whitney JT3D engines. Aeronautic Development Corp. Ltd. was successful in devising a quiet nacelle system and hush kit to make those airplanes economically viable. That system and hush kit is shown in allowed application Ser. No. 06/769,141, now U.S. Pat. No. 4,723,626.
Many municipalities and airports have now adopted the more stringent FAR 36, Stage 3, noise level requirements. Those regulations affect the usefulness and value of a much greater number of aircraft, including particularly Boeing 727-200 aircraft having Pratt & Whitney JT8D engines. About 1,700 of those airplanes currently are in operation. Heretofore, no one has successfully developed a system and hush kit to extend the useful life of those airplanes.
It is a difficult task to make those existing airplanes meet the stage 3 noise level requirements. Studies which have been conducted show that noise radiates from a low bypass, fan jet engine in several directions. High-frequency fan noise radiates both forward though the air inlet cowl and afterward through the exhaust ducts. Low-frequency jet noise generally radiates rearwardly. At low engine thrust, the high-pitch whine of the fan is more pronounced. At high engine thrust, the low-pitch jet rumble is more noticeable. Each of those noise components must be dealt with separately, as well as in combination. But, making a modification to suppress noise may also severely degrade airplane performance, substantially increase final consumption, require longer runways for take-offs or landings, mandate lesser payloads or otherwise greatly increase costs or require operational limitations.